1. Field of the Invention
The invention relates to coating formulations, more particularly topcoat and clearcoat materials, which comprise particles which on their surface have protected isocyanate groups.
2. Description of the Related Art
Coating systems comprising particles—more particularly nanoparticles—are state of the art. Such coatings are described for example in EP 1 249 470 or WO 03/16370. The particles in these coatings lead to an improvement in the properties of the coatings, more particularly with regard to their scratch resistance and also their chemical resistance.
A frequently occurring problem associated with the use of the—generally inorganic—particles in organic coating systems consists in a usually inadequate compatibility between particle and coating-material matrix. This can lead to the particles being insufficiently dispersible in the matrix. Moreover, even well-dispersed particles may undergo settling in the course of prolonged standing or storage times, with the formation, possibly, of larger aggregates or agglomerates, which even on redispersion are then impossible or difficult to separate into the original particles. The processing of such inhomogeneous systems is extremely difficult in any case, and in fact is often impossible. Coating materials which, once applied and cured, possess smooth surfaces are generally preparable by this route not at all or only at great cost.
Favorable, therefore, is the use of particles which on their surface possess organic groups that lead to improved compatibility with the coating-material matrix. In this way the inorganic particle becomes “masked” by an organic shell. Particularly favorable coating-material properties can be achieved in this context if, furthermore, the organic functions on the particle surfaces also possess groups that are reactive toward the coating-material matrix, so that under the respective curing conditions of the coating material in question they are able to react with the matrix. In this way, success is achieved in incorporating the particles into the matrix chemically in the course of coating-material curing, which often may result in particularly good mechanical properties but also result in improved chemical resistance. Systems of this kind are described for example in DE 102 47 359 A1, EP 832 947 A or EP 0 872 500 A1.
Also known, furthermore, is the use of coatings comprising a binder which has been modified with nanoparticles. These coatings can be produced by reacting the particles, equipped with a reactive functionality, with a binder containing a complementary reactive functionality. In this case, therefore, the organofunctional particle is incorporated chemically into the coating-material matrix not only at the coating-material curing stage but also even at the binder preparation stage. Systems of this kind are described for example in EP 1 187 885 A or WO 01/05897.
In the case of one particularly important type of coating material, a film-forming resin is used which comprises hydroxy-functional prepolymers which, on curing of the coating material, are reacted with an isocyanate-functional curative. These polyurethane coating materials are notable for particularly good properties, such as a superior chemical resistance, for example, yet there is still a need for improvement in particular as regards the scratch resistance of these systems. Typically they are used in particularly high-value and demanding fields of application: for example, as clearcoat and/or topcoat materials for OEM paint systems in the automobile and vehicle industry. The majority of refinish coating materials, for automobile repairs, are also composed of isocyanate-curing systems of this kind.
Typically a distinction is made between two different polyurethane coating systems, known as 2K and 1K systems. The former consist of two components, one of which is composed essentially of the isocyanate curative, while the film-forming resin with its isocyanate-reactive groups is contained in the second component. The two components must be stored and transported separately and should not be mixed until shortly before they are processed, since the potlife of the completed mixture is greatly limited. Often more favorable, therefore, are the so-called 1K systems, composed of just one component, in which alongside the film-forming resin there is a curative containing protected isocyanate groups. 1K coating materials are cured thermally, the protective groups of the isocyanate units being eliminated, with the deprotected isocyanates being able then to react with the film-forming resin. Typical baking temperatures of such 1K coating materials are situated at 130-160° C.
In the case of these high-value coating materials a further improvement in properties would be desirable. This is true in particular of vehicle finishes. For instance, the achievable scratch resistance of conventional autofinishes, in particular, is still not sufficient, with the consequence, for example, that particles in the washwater in a carwash lead to significant marring of the finish. Over time, this causes lasting damage to the gloss of the finish. In this situation, formulations that allow higher scratch resistances to be achieved would be desirable.
One particularly advantageous way of achieving this objective is to use particles having protected isocyanate functions on their surface. Where such particles are incorporated into 1K polyurethane coating materials, the isocyanate functions on the particle surfaces are liberated as well in the course of coating-material curing, and the particle is incorporated chemically into the finish. Moreover, the protected isocyanate functions enhance compatibility between particle and coating-material matrix.
Particles of this kind containing protected isocyanate functions are in principle already known. Typically they are prepared by condensing particles having free silicon or metal hydroxide functions with alkoxysilyl-functional organosilicon compounds whose organic radical contains protected isocyanate functions. Organosilicon compounds of this kind containing masked isocyanate groups have already been described, as in DE 34 24 534 A1, EP 0 212 058 B1, JP 08-291186 or JP 10-067787, for example. The particles containing protected isocyanate functions themselves, and their use in coatings, are described in EP 872 500 A, for example.
The scratch resistance of coatings can in fact be increased significantly through the incorporation of such particles. However, in all of the methods of using these particles that have been described in the prior art, optimum results have still not been achieved.